test: drop extra then() handler in header lookup

The must-call-assert rule allows only one argument to Promise.then().

Signed-off-by: Yagiz Nizipli <yagiz@nizipli.com>

Author: anonrig

test/parallel/test-http-server-raw-header-lookup.js

@@ -59,5 +59,5 @@ server.listen(0, common.mustCall(() => {
     assert.match(missingHost, /^HTTP\/1\.1 400 /);
 
     server.close();
-  })().then(common.mustCall(), common.mustNotCall());
+  })().then(common.mustCall());
 }));